1. Field of the Invention
This invention relates to the protection of MR/GMR magnetic read heads from electrostatic discharge and dielectric breakdown. It also relates to the reduction of noise in the readback signal during operation of MR/GMR magnetic read heads.
2. Description of the Related Art
Magnetic read heads utilizing magnetoresistive or giant magnetoresistive (MR/GMR) sensor elements are used in the retrieval of magnetically stored data from moving magnetic media such as tapes and disks. Such read heads are formed integrally with a xe2x80x9cslider,xe2x80x9d which literally flies over the surface of the moving medium on a layer of air. The sensing portion of the read head consists of the MR/GMR sensor element, sandwiched between two magnetic shields that are insulated from the sensor by thin dielectric layers and whose purpose is to isolate the sensor from the magnetic fields of nearby encoded tracks that are not being accessed. A constant biasing current must be passed through the sensor element during its operation in order to convert its resistance variations to a readable signal.
During their operation, such read heads are subject to various forms of readback noise. Not only does such noise degrade the quality of the signal produced by the head, but the various causes of the noise can also damage the head itself. One source of readback noise is the buildup of electrical charge on various parts of the head as a result of friction and the relative motion between the head and the magnetic medium. When sufficient charge accumulates, the electrostatic potential difference between parts of the head, notably the sensor element and magnetic shields, can exceed the dielectric breakdown strength of the insulating materials between these parts, causing a sudden electrical discharge. This discharge produces noise, sometimes called xe2x80x9cspike noise,xe2x80x9d in the signal and can damage the parts of the head which receive it. The invention of Sato et al. (U. S. Pat. No. 4,802,043) teaches a method of forming a magneto-resistive read head with partially surrounding conducting layers that remove the static electric charge buildup before damaging discharges can occur. The method of this invention requires a permanent connection, with negligible resistance, from the ground lead of the magnetoresistive sensor to both shields. Denison et al. (U.S. Pat. No. 5,539,598) teach a method for forming a magneto-resistive read head whose magnetic shield is permanently coupled to ground through a resistor of several kilo-ohms. Finally, Hughbanks et al. (U.S. Pat. No. 5,761,009) teach a method for forming a parasitic shield in close proximity to the actual magnetic shield of the read head. By maintaining the parasitic shield at the same potential as the sensor element of the read head, yet having it in closer proximity to the actual magnetic shield, electric discharges will preferentially be from shield to shield, rather than shield to sensor.
There is yet another cause of electrical discharge between the magnetoresistive sensor and its surroundings that is unrelated to the frictional buildup of static electric charges. As was discussed above, in the normal operation of a magnetoresistive read head a longitudinal bias current passes through the sensor element, entering and leaving through conductive leads that are affixed to it. This current produces an Ohm""s law potential variation along the length of the sensor. Since the magnetic shields do not carry such a current, they are equipotential surfaces. Therefore, with the possible exception of a single small region, there will exist a significant potential difference between shield and sensor. FIG. 1 graphically illustrates the potential variation along a sensor with nominal 50 ohm resistance and a bias current of 5.0 milliamps, as compared with the constant potential of its magnetic shield. As can be seen, the greatest potential difference between shield and sensor occurs where the shield is in closest proximity to the sensor/lead junction, where the bias current enters and leaves the sensor. It is at these points that electrical discharges can occur between the shield and the sensor, leading to noise and possibly to damage. It has been noted that this type of electrical discharge occurs in an irregularly oscillating or xe2x80x9csee-sawxe2x80x9d pattern. When the discharge occurs at one end of the shield, the transfer of charge causes the shield potential to suddenly change. This makes the other end of the shield the point of maximum potential difference between it and the sensor. The next discharge is, therefore, most likely to be at that end. Thus, the pattern of end to end discharges continues. The severity of this type of discharge is exacerbated by metal xe2x80x9csmearingxe2x80x9d on the air-bearing surface (ABS) of the read head. If the metal smear on a noisy head is removed by focused ion-beam (FIB) etching, the noise is reduced. None of the previously cited inventions addresses this form of electrical discharge.
It is an object of the present invention to significantly reduce the frequency and severity of readback noise caused by electrical discharges due to bias-current longitudinal potential differences between a MR/GMR sensor and its magnetic shields.
It is another object of the present invention to significantly reduce the possibility of damage to a MR/GMR sensor caused by electrical discharges due to bias-current longitudinal potential differences between said sensor and its magnetic shields.
It is yet another object of the present invention to provide a method for fabricating an MR/GMR magnetic read head, during whose operation there is a reduction in the frequency and severity of readback noise and in the possibility of damage, both caused by electrical discharges due to bias-current longitudinal potential differences between its MR/GMR sensor and its magnetic shields.
It is yet another object of the present invention to provide an MR/GMR magnetic read head, during whose operation there is a reduction in the frequency and severity of readback noise and in the possibility of damage, both caused by electrical discharges due to bias-current longitudinal potential differences between its MR/GMR sensor and its magnetic shields.
It is yet another object of the present invention to provide a mechanism for reducing both the frequency and severity of readback noise in a MR/GMR sensor and the possibility of its damage, caused by electrical discharges due to bias-current longitudinal potential differences between its MR/GMR sensor and its magnetic shields, wherein such mechanism also permits diagnosis of inadvertent sensor-to-shield short circuits and helps in the failure analysis of open circuits during the fabrication process.
It is yet another object of the present invention, by providing a mechanism for reducing both the frequency and severity of readback noise in a MR/GMR sensor and the possibility of its damage due to bias-current longitudinal potential differences between its MR/GMR sensor and its magnetic shields, to create a slider assembly that is less sensitive to xe2x80x9cmetal smearxe2x80x9d at the air-bearing surface (ABS) and, thereby, to increase the slider fabrication yield.
The objects of this invention are achieved by a read head structure and method of fabrication that allows the magnetic shields of the read head to be held at the same potential as the center of the MR/GMR sensor element. Because of the extremely small size of the sensor element, it is impractical to electrically connect it directly to the shields. Accordingly, the present invention teaches a method of fabricating a balanced electrical half-bridge of large surface area and substantially greater resistance than the MR/GMR sensor element that is connected in parallel with said sensor. The mid-point of the half-bridge is of sufficient size to permit connection to the shields and it has a pad for forming interconnects with said shields. Said half-bridge has a resistance which is approximately 100xc3x97 the sensor resistance and, therefore, dissipates that much less power than the sensor. Its greater surface area allows it dissipate more heat in steady state operation and to store more heat during a transient. Therefore, the half-bridge is in no danger of melt-down during either steady state or transient operation.
The half-bridge also assists in failure mode analysis of the MR/GMR sensor. If said MR/GMR sensor is destroyed by electrostatic discharge (ESD) or overlapped so as to destroy the MR/GMR structure during slider fabrication, the known intermediate value of the measured resistance with half-bridge in place will be on the order of 5,000 ohms. If, on the other hand, the open is due to a broken wire or bad bonding, the measured resistance will be infinite. Thus, the electrical half-bridge provides a mechanism for discriminating between two failure modes that, in the present art, are indistinguishable by resistance measurements. Finally, the present invention achieves the above objects in a manner that allows the connection between the MR/GMR sensor and the shields to remain permanently in a finished slider, because any noise from the shields is rejected by a differential preamplifier. It is to be noted that the methods taught in U.S. Pat. Nos. 4,802,043 and 5,539,598, will not work with differential preamplifiers because there is no ground lead. The parasitic shields formed in accordance with the invention of Hughbanks (U. S. Pat. No. 5,761,009) are positioned further from the high potential ends of the sensor element than the actual shields and would not offer protection from the type of discharges that are the focus of the present invention.